Abstract
More than 70% of Pelvetia fastigiata eggs and about 15% of Fucus distichus eggs become polyspermic when fertilized at natural sperm concentrations in a low-sodium (2.5 m M Na +, 450 m M N-methyl glucamine) artificial seawater. Natural levels of polyspermy are 1–3% for both species. Polyspermic eggs germinate and respond to photopolarization, but do not develop beyond an abnormal, “stumpy,” four-cell stage. They die within 1–1.5 weeks. The sodium-dependent block is a fast block, and it is replaced by a second block (probably cell wall formation) no later than 9 min ( Pelvetia) after eggs are shed. The sodium-dependent block in Pelvetia is very efficient; when external sodium is raised to only 47.5 m M, the level of polyspermy drops to about 25%. These results are compared with data on marine invertebrates in the context of factors such as the sperm/egg concentration at fertilization and natural, osmotic (salinity) stress.
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